Young children age 3-5.5 years will be studied. The study includes treatment with different doses of methylphenidate and placebo; all children will receive active medication during the study. Prior to treatment, each child will receive a thorough psychiatric evaluation and each guardian will complete a 10-week workshop (2 hours/week). Throughout the study, parents and teachers will complete forms that report on the child's behavior and possible side effects. Participants will be monitored by regular visits with a study physician. Most participants will be involved in the study for up to 16 months to ensure optimal evaluation, dosing, and monitoring